1. Field of the Invention
This invention relates to a magnetic recording medium, more particularly to a graded magnetic recording medium.
2. Description of the Related Art
In recent years, demand for data storage has continuously increased. To meet present requirements, not only must a grain size in high density recording media be reduced to increase the recording density, but a good isolation must also be provided to reduce noise interference between grains generated by intergranular coupling. In addition, a sufficient perpendicular magnetic anisotropic energy (Ku) is also required to provide a good thermal stability (KuV/kT). However, if the volume of the grain is too small, the product of the magnetic anisotropic energy and the volume (KuV) will be insufficient to overcome the thermal disturbance caused by external temperature, thereby resulting in an unstable magnetic moment known as superparamagnetism.
In a perpendicular type recording medium with a small recording bit size, since the direction of the magnetic moment is perpendicular to a layer surface direction of a magnetic recording layer, the stability thereof will not be adversely affected. Therefore, in the existing technologies, the perpendicular recording type is generally used to increase the magnetic anisotropic energy (Ku) so as to achieve an improved thermal stability.
In addition, the ferromagnetic property of a magnetic recording layer of a graded recording medium is increased gradually from an upper surface to a bottom surface thereof. That is, the ferromagnetic property is gradually changed from a soft magnetic property to a hard magnetic property from the upper surface to the bottom surface of the magnetic recording layer, so that the overall writing field can be reduced. Further, the magnetic recording layer of the graded recording medium may maintain the required thermal stability due to its hard magnetic property.
Jai-Lin Tsai et al. disclosed a method for making a graded Fe/FePt film (“Magnetic properties and microstructure of graded Fe/FePt films”, JOURNAL OF APPLIED PHYSICS 107, 113923 (2010)). Referring to FIG. 1, the method comprises forming a Fe56Pt44 alloy layer 12 having a thickness of 10 nm on a glass substrate 11 by a DC magnetron sputtering process. Next, the glass substrate 11 formed with the Fe56Pt44 alloy layer 12 is subjected to a rapid thermal process (RTP) at 800° C. for 10 minutes to change the Fe56Pt44 alloy layer 12 into an ordered phase (L10 phase, a face-centered tetragonal (fct) structure) with a hard magnetic property. Thereafter, a Fe layer 13 having a thickness of 6 nm is sputtered on the Fe56Pt44 alloy layer 12, followed by subjecting the Fe layer 13 to the rapid thermal process at a temperature of 700° C. for 1 minute to allow interdiffusion between the Fe atoms of the Fe layer 13 and the Pt atoms of the Fe56Pt44 alloy layer 12, thereby changing the Fe56Pt44 alloy layer 12 and the Fe layer 13 into a graded Fe/FePt alloy film 14. Finally, a SiO2 layer 15 is formed on the graded Fe/FePt alloy film 14.
The ferromagnetic property of the graded Fe/FePt alloy film 14 is gradually increased from an upper surface 141 to a bottom surface 142 thereof. However, the temperature required to form the FePt alloy of the L10 phase in the rapid thermal process is up to 700° C.˜800° C. Therefore, the high process temperature is not suitable for applying to elements that have been incorporated into a semiconductor device.
C. L. Zha et al. disclosed a method for making a magnetic recording medium (“Continuously graded anisotropy in single (Fe53Pt47)100-xCux films”, APPLIED PHYSICS LETTERS 97, 182504 (2010)). Referring to FIG. 2, the method comprises first forming a SiO2 layer 22 having a thickness of 1 μm on a silicon (Si) substrate 21 by a thermal oxidation process. Next, a Ta layer 23 having a thickness of 6 nm and a Pt layer 24 having a thickness of 3 nm are formed in this order on the SiO2 layer 22, followed by forming a (Fe53Pt47)100-xCux alloy film 25 having a thickness of 20 nm on the Pt layer 24 using a co-sputtering process. In the (Fe53Pt47)100-xCux alloy film 25, the Cu content is gradually decreased from a bottom surface 251 to an upper surface 252 thereof, so that the compositions of the bottom surface 251 and the upper surface 252 are (Fe53Pt47)70Cu30 and Fe53Pt47 respectively. The gradient variation of the Cu content is achieved by controlling the output power of the Cu target in the co-sputtering process. Thereafter, the (Fe53Pt47)100-xCux alloy film 25 is subjected to an annealing process at a temperature of 500° C. for 35 minutes to change the (Fe53Pt47)100-xCux alloy film 25 into a graded magnetic recording film 26. Finally, a capping layer 27 that is made of a Ta material and that has a thickness of 5 nm is formed on the graded magnetic recording film 26 to prevent oxidation of the graded magnetic recording film 26.
C. L. Zha et al. utilizes the doping of Cu atoms to reduce the ordering temperature required to form the FePt alloy of the L1o phase. Thus, the graded magnetic recording film 26 has L1o phase adjacent to a bottom surface 261 thereof and the disordered phase (also referred to as A1 phase) adjacent to an upper surface 262 thereof, to form a face-centered cubic (fcc) structure. Therefore, the ferromagnetic property of the graded magnetic recording film 26 is gradually increased from the upper surface 262 to the bottom surface 261. In this way, the graded magnetic recording film 26 may maintain a process temperature at about 500° C. to meet the requirement of the manufacturing process of semiconductor devices. However, it is found that, through the analysis of X-ray diffraction (XRD), the graded magnetic recording film 26 has only (111) preferential orientation, and lacks (001) preferential orientation that is necessary for graded perpendicular recording media with high recording density. Thus, perpendicular magnetic anisotropy and thermal stability are somewhat adversely affected. Therefore, there is a need in the art to provide a magnetic recording medium that has an improved perpendicular magnetic anisotropy and thermal stability.